1. Field of the Invention
The present invention relates to rolling bearings for use in an auxiliary device such as an alternator or the like mounted on an internal combustion engine (engine) and, more particularly, to a technology of preventing brittle flaking (white-banded flaking) of a rolling bearing.
2. Description of the Related Art
Rolling bearings for auxiliary devices such as an alternator of an automotive engine, an air conditioning unit and idler pulleys have heretofore been used in the past under severe conditions involved in vibrations and temperatures. This results in the elicitation of flaking in the rolling bearings due to a change of tissues in new modes. This flaking occurs in any area of an outer race, an inner race and a rolling element (ball or roller) of the rolling bearing under a feature that is different from a usual rolling fatigue life encountered in the related art. Once such a phenomenon occurs, flaking occurs for a very short of time (a time interval of approximately 1/100 to 1/1000 times that of the related art). In observed tissue of a flaked area after etching with nital liquid (nital alcoholic solution), as done in the observation of the fatigue life in the related art, the flaked area has features with the tissues seemed not to be dark (in so-called DEA: Dark Etching Area) but instead a white layer (in a so-called WEA: White Etching Area).
In a bearing industry, it is a usual practice for this flaking to be referred to as “brittle flaking” or “white-banded flaking” in order to be distinguished from the related art fatigue life. In a rolling life test conducted so as to cause the occurrence of flaking using the related art fatigue test, a whole of component elements of a rolling bearing undergoes fatigue breakdown in a final stage. In contrast to such fatigue breakdown, no mechanism of the occurrence of brittle flaking is made clear yet and such flaking shows specific properties with breakage taking place depending on recurrence test conditions under which flaking takes place for an extremely short period of time whereas under a condition with no occurrence of breakage, brittle flaking does not occur. Therefore, attempt has heretofore been made in the past to address such an issue through merely temporary treatment conducted with no clear scientific grounds and in such a status, no full-fledged measures can be taken under current conditions.
Such a mechanism is explained on presently accepted hydrogen theory. This is a theory in that when a ball is caused to slip due to stress such as vibration during use thereof causing grease to be decomposed due to heat or pressure developments for generating hydrogen, which in turn causes hydrogen brittleness to occur with the resultant occurrence of flaking. Various proposal have heretofore been made with a view to suppressing the separation of hydrogen from grease or providing a rolling surface coated with an oxide film so as to shut off the entry of generated hydrogen into steel of a rolling bearing for thereby suppressing the occurrence of flaking based on such theory as disclosed in Japanese Patent Publication No. 6-89783. However, experimental test results obtained by the present invention have not necessarily demonstrated a solution to the issue of preventing the occurrence of flaking. Also, upon recurrence tests conducted under another condition different from that under which an effective result is obtained, no advantageous effect has been obtained but rather a worse phenomenon has appeared. Although in a case where a rolling bearing is made up of component elements made of steel forcedly added with hydrogen in advance, white-banded flaking surely affects the rolling bearing for a short period of time even if most of tests have been conducted under any conditions. No conclusion has been obtained in normal operation of the rolling bearing in that grease is decomposed into hydrogen for penetration into steel to cause the occurrence of white-banded flaking due to hydrogen brittleness.
Further, the present inventor has conducted a test using ball bearings under a condition shown in FIG. 1 to demonstrate such a phenomenon on the ball bearings in a more reliable manner. With the test, three kinds of greases were employed in the ball bearings, respectively, as indicated on Table 1 shown in FIG. 11. Greases A and C include compounds having hydrogen radical and grease B includes grease of fluorine series with no hydrogen radical.
The test condition is illustrated in FIG. 1 and has been properly conducted under a condition with no entry of moisture into the ball bearings from an outside. According to the hydrogen theory, white-banded flaking would not occur on the ball bearing with grease B, but white-banded flaking has eventually occurred on this ball bearing. Also, this flaking has occurred for a shorter time interval than that for which flaking had occurred on the ball bearing with grease C having hydrogen radical. (However, it is likely that there is an effective sequence opposite to that of FIG. 1 depending on a test condition. Now, what the inventor desires to state a fact that white-banded flaking has actually occurred on the ball bearing even with grease B with no inclusion of hydrogen radical.) That is, it is concluded that hydrogen is not the cause of white-banded flaking.
Due to the presence of strong impression on test results (with hydrogen brittleness) conducted on the bearings each with the rolling surface forcedly added with hydrogen, the favorable test results appearing by chance have been mistaken to be a hydrogen ingress prevention effect resulting from the formation of the oxide film on the rolling surface. In other word, although it is hard to deny a possibility in that hydrogen constitutes just a little bit accelerating factor, hydrogen is not a main factor for the occurrence of white-banded flaking.
In fact, even with a real machine test conducted on an automobile on which bearings taking measure based on such a theory, flaking has occurred on the bearings. Recurrence tests, conducted by the present inventor, have come to a conclusion in that those of the bearings encountered with white-banded flaking, resulting not from hydrogen but from some reason (other than hydrogen), have included some of the bearings in which hydrogen is eventually observed in steel. That is, this has resulted from confusion made between causes and errors.
As a further mechanism, an attempt is made to use a stress theory (an oscillation theory in terms of stress). That is, this concept is an idea to explain flaking in terms of the stress side. Such a concept similarly results in contradiction in which it becomes hard to distinguish the occurrence of a common fatigue life (accompanied by DEA) based on stress (shear). Moreover, the present inventor has determined the fact, upon various researches conducted on stress acting on the bearings, in that among those of the bearings encountered with flaking in the real machine (automobile) test, some of the bearings are encountered with white-banded flaking when the load, acting on the bearing, clearly drops to 0 Kg when a belt tension is zeroed (becomes less than 0 Kg) due to adverse affect resulting from an inertial force during deceleration of the engine under a condition in which the belt tension remains at a low level. There is no way for the stress theory to explain such an exemplary case. Although other explanations are herein omitted, the stress theory has the same contradiction as that of the hydrogen theory discussed above and, therefore, white-banded flaking stills occur on bearings installed on a real machine for which measures have been taken based on such a theory.
As set forth above, any mechanism propounded in the related art for the occurrence of brittle flaking does not satisfy the current condition and still remains to be unclear about what is a stress factor among other factors of a real machine and how the stress factor gives adverse affect on the bearing. Therefore, there is a situation under which no measure is taken to address the issues. Further, modern engines have heretofore been widely incorporating serpentine type belt drive systems with each including a single belt for driving a large number of pulleys with a view to structuring the engine in a small size with lightweight. Such a belt drive system has encountered with various issues such as an increase in tension of a belt, a resonance on the belt and enhanced engine oscillations. Thus, stress acting on the bearings has been increasingly complicated. No measure has been taken to address even such issues. Especially, among the auxiliary devices installed on the automobile, an alternator is a typical unit in which flaking occurs in the bearing at an increased rate as compared to the bearings of the other the auxiliary devices. Even though the bearings constitute important mechanical component parts, no full-fledged measure has been taken to address the issue of the occurrence of brittle flaking in a rolling bearing and the rolling bearing is still left under a situation where even no occurrence mechanism has been established.
As mentioned above, despite the occurrence of brittle flaking causes extremely shortened life as compared to a commonly occurring fatigue life (for instance, no fatigue life has been serious issues on an alternator in actual practice), no mechanism for explaining such a phenomenon has been established and no proper measures have been taken. With no alternatives, attempts have heretofore been taken to use an inefficient method wherein inconsistent measures have been taken for respective auxiliary devices to confirm results on tests of the auxiliary devices. Therefore, wasteful efforts have been conducted unnecessarily increasing a size or precision of the bearing and even with such efforts, such problems cannot be fully addressed.